Optical monitoring of optical signals is very important in ensuring the correct operation of optical communications networks. The relevant standard for optical monitoring is set out in the International Telecommunications Union document ITU-T G.697 “Optical monitoring for DWDM systems SERIES G: TRANSMISSION SYSTEMS AND MEDIA, DIGITAL SYSTEMS AND NETWORKS Transmission media characteristics—Characteristics of optical components and subsystems”.
Optical monitoring points in a communications network typically comprise an optical splitter arranged to extract between 1% and 10% of the optical signal that is to be monitored, the extracted signal being provided to a monitoring port. All of the traffic carried by the optical signal being monitored is replicated in the extracted signal and is provided to the monitoring port. There is a resulting problem that live traffic is vulnerable to eavesdropping at the monitoring port and this presents a problem of communications network security.
International Telecommunications Union document ITU-T X.805 “Security architecture for systems providing end-to-end communications” sets out various optical protection schemes for making an optical connection secure against a fibre being cut to place an in-line tap for eavesdropping. However, the methods set out in ITU-T X.805 only monitor cuts in an optical communications network fibre link and are not able to detect eavesdropping of an optical signal via a monitoring port.
An alternative approach that has been proposed for providing communications security is optical encryption of the signals transmitted across an optical communications network, as proposed by Jung et al, “Demonstration of 10 Gbps all-optical encryption and decryption system utilizing SOA XOR logic gates”, Optical and Quantum Electronics, vol. 40, no. 5-6, April 2008. A problem faced by optical encryption is that optical encryption and decryption devices are required for each wavelength channel at each transmitter and receiver within a communications network, raising the cost of the network.